In commonly owned U.S. Pat. No. 3,988,547 there has been disclosed a telecommunication system of this type, including a central office or exchange adapted to handle both dial pulses and multifrequency signals. As further described in that patent, a keyboard on a telephone set designed for pushbutton selection has an extra pushbutton whose operation characterizes the accompanying or immediately following frequency combinations as data signals rather than selection signals.
Such extra pushbuttons or equivalent switch means are widely used in practice to prevent the response of central-office equipment to spurious frequency combinations due, for example, to the transmission of sound waves from a nearby radio or from a receiver of piped musical programs. Usually, the pushbutton generates a d-c signal (referred to hereinafter as a key signal) which travels together with the selection frequencies over the associated line loop to a peripheral unit of the exchange and which may take the form of a diminution of the line current normally traversing the loop, brought about for example by the insertion of an additional resistance.
In the case of outlying subscribers communicating with a remote exchange via a radio link or some other channel incapable of transmitting direct currents, such a key signal cannot be sent along with the selection frequencies. Thus, the protection afforded thereby against spurious operations is unavailable in this type of system.